<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0px;
            padding: 0px;
        }
        .box{
            width: 300px;
            margin: 0 auto;
            overflow: hidden;
        }
        ul{
            list-style: none;
        }
        .box .item{
            float: left;
            width: 148px;
            text-align: center;
            border: 1px solid blue;
            background: blue;
            color:white;
            line-height: 40px;

            position: relative;
        }
        .item:hover{
            background: lightblue;
            color: black;
        }
        .item>ul{
            display: none;
            background: white;
            color: black;

            position: absolute;

            left: 0px;
            top: 40px;

            width: 148px;
        }
        .item:hover ul{
            display: block;
        }
        .item li:hover{
            color: blue;
        }
        .content{
            height: 100px;
            background-color: yellow;
        }
    </style>
</head>
<body>
    <ul class="box">
        <li class="item">视频教程
            <ul>
                <li>全部视频教程</li>
                <li>HTML5视频教程</li>
                <li>JAVA视频教程</li>
                <li>Python视频教程</li>
            </ul>
        </li>
        <li class="item">认证考试
            <ul>
                <li>11111</li>
                <li>22222</li>
                <li>33333</li>
            </ul>
        </li>
    </ul>

    <div class="content">11111111111111111</div>
</body>
</html>